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So many studies emphasize the Virginia front that any good book on the Civil War in the West is noteworthy. Any book claiming battles in the West decided the war demands special attention. Such is “Where the South Lost the Civil War,” by Kendall Gott, a military historian who is unafraid (like his hero Ulysses S. Grant) of daring assertiveness. But does he prove his point?

Aided by ironclads operating on Kentucky and northern Tennessee rivers, Grant took Forts Henry and Donelson in February 1862. Confederate prisoners exceeded 12,000. Worse, Confederate armies had to abandon central Kentucky to the east and, to the west, their impregnable position at Columbus, on the Mississippi. The Confederacy never held Tennessee again; its other Western states were forever after vulnerable. “It was here,” Mr. Gott says, “the inexorable decline of the Confederacy began.”

Mr. Gott writes well about strategic issues on the Western front. Jefferson Davis’ decision to defend all Confederate territory had different consequences in the West than the East, where the narrow Washington-Richmond corridor and the northwest-southeast flow of rivers toward the Atlantic was an advantage to the defense. In the West, there was too much land to defend — as well as rivers, such as the Cumberland and the Tennessee, whose north-south angles made them natural routes for invasion. Given Grant’s skill in coordinating ground actions with new ironclads commanded by Andrew Hull Foote, geography made the Southern heartland in the West far more vulnerable.

Mr. Gott also provides strong, well-researched portraits of leaders and leadership, not just Grant, but his subordinate Lew Wallace, who kept his head during a severe counterattack at Fort Donelson, and the Southern cavalry genius Nathan Bedford Forrest, whose legend began there. The author also provides fresh sketches of nonmilitary figures, such as St. Louis shipbuilder James B. Eads, who basically created the Federal inland navy.

Mr. Gott shows that Federal soldiers fought well when well led — as they were in the West — while stumbling (until the middle of the war) in the East. Grant ranks first among the Union generals.

A West Point enrollment mistake made Hiram Ulysses into “Ulysses Simpson” Grant; victory at Fort Donelson gave him the nickname “Unconditional Surrender.” Grant became a press hero for sending such terms to the last Fort Donelson commander, Simon Bolivar Buckner, who called them “ungenerous and unchivalrous.”

Mr. Gott sagely notes, however, that Grant’s actual treatment of prisoners was quite courteous, foreshadowing Appomattox. This was vital, the author argues. Early in the war, without guidance, some Union commanders wanted punitive treatment of “rebels.” Grant’s courtesy to POWs sent a seductive message, Mr. Gott observes: Surrender to this man was no disaster.

Bitter rivalry divided Southern generals, especially Buckner and Gideon Pillow, whose rash actions, at odd times brilliant, alienated many. Both were commanded by John B. Floyd, secretary of war under President James Buchanan. The Federals wanted Floyd arrested for shipping munitions south before the war began. The trio mismanaged the defense of Donelson, which Mr. Gott terms a “fiasco.”

He reserves highest blame — as did Southern newspapers — for department commander Albert Sidney Johnston, who let the forts become isolated. Johnston was a fine field general (as he proved gallantly at Shiloh in April 1862), but not a man to run a regional headquarters, Mr. Gott contends. He implies that Grant’s main nemesis was his own superior, Henry “Old Brains” Halleck, whose meticulous planning held Grant back.

Mr. Gott is excellent on Grant’s ironclad navy, detailing how, between attacks on the two forts, Union gunboats raided far up the Tennessee into Alabama, destroying boats, supplies and bridges as they steamed. On the waters, he says, Southern leaders “quite simply became shaken by the use of a relatively new technology.”

Despite strengths, Mr. Gott’s work has problems. He is sometimes repetitive, which is not helpful in a work of this length. The narrative is not always smooth. Good maps illustrate major actions and break up the text, but there are stretches of gray prose that some reader-friendly subheads could have made more accessible.

Mr. Gott’s basic claim is a tad excessive. The West was critical, and taking the forts was a first step toward taking Atlanta two years later. But this does not mean the South lost the war at the forts. Mr. Gott justifies his claim in part: Early Union victories in Tennessee were critical to final victory. But in a razor-thin win like this, every Union victory was in truth indispensable. Just subtract Grant’s recovery from Johnston’s later surprise at Shiloh, and the North’s chances look dim — as Mr. Gott concedes near the end.

Overemphasizing the forts also undermines Mr. Gott’s own appreciation of the fact that the war was a political as well as a military struggle. He accurately notes how victory at the forts boosted Northern morale after defeats such as Bull Run in 1861. But he could turn this around: In 1863, what would Federal defeat on Cemetery Ridge have done? Or the complete destruction of Joseph Hooker at Chancellorsville? Aim Southern artillery lower at Gettysburg, or aim Northern artillery (the only thing that held) too high in the Wilderness, and Washington might have whistled Dixie, and London recognized Richmond, whatever happened out West. Even with William T. Sherman in Georgia during the summer of 1864, Lincoln’s re-election was by no means assured; stalemate in Virginia didn’t help. The fate of the Union was dicey for three years.

Taking the forts was vital but did not decide the war. At many later points, the Union might have lost or the South might have won — mostly, despite the claims of critics who focus on casualties, because of Robert E. Lee. (If nothing else, Mr. Gott’s focus on Union success in the West should suggest how formidable Lee’s generalship was in the East.) This is a solid book, but the title and claims go “a fort too far.”